Running an apt mirror now is fine. If any issues are found in Linux Mint 18 beta the fixes to packages will go into the repository as well as onto the final ISO release. So as long as you'll keep the mirror in sync, it's fine.

The same goes BTW for using the Linux Mint 18 beta. For enthusiasts, early adopters, or those that just can't wait as long as they keep up with updates their installation will be upgraded to the final release through Update Manager. If there is anything more to do to upgrade from beta to final release it will be noted in the final release announcement. I can recall only one release over the past 5 years where you needed to do anything more than apply level 1-2 upgrades in Update Manager.